This invention, which is a result of a contract with the United States Department of Energy, relates generally to the art of magnetic field confinement of plasmas in closed magnetic field line devices and more specifically to improvements in the containment field geometry of an Elmo bumpy type plasma confinement device.
One form of a closed magnetic field line plasma confinement device is an Elmo bumpy torus which may be referred to as one of a number of linked toroidal closure plasma devices in which a plurality of magnetic mirror type confinement segments are linked to form a torus. By linking the segments together, plasma escaping from the ends of one mirror sector is not lost, as in a simple open ended mirror device, because it enters an adjoining mirror sector.
Unfortunately, plasmas confined simply by toroidally linked mirror devices do not have magnetohydradynamic (MHD) stability. However, in an Elmo bumpy torus (EBT) MHD instability has been overcome by periodic spatial modulation of the magnetic field, the so-called "bumps" in the confinement field and by surrounding each bump with a microwave cavity supplied with radiofrequency energy to form a ring of electrons which rotate about the confined plasma in the bump. Electric currents generated by these rings produce changes in the magnet field that provide gross MHD stability to the confined central plasma. Details of a stabilized bumpy torus plasma confinement device of this type may be had by referring to U.S. Pat. No. 3,728,217 issued Apr. 17, 1973 to R. A. Dandl, the subject matter of which is incorporated herein by reference thereto.
Several aspects of the EBT concept make it attractive as a fusion device: the large aspect ratio (major radius of the plasma torus R/the minor radius r), noninterlocking circular field windings with modest field; and the steady-state operation. However, one of the major concerns with the conventional toroidal geometry of the EBT is the problem associated with plasma particle confinement in curved magnetic fields. Results of recent plasma confinement calculations for and preliminary reactor assessments of advanced bumpy torus configurations has been summarized in an Oak Ridge National Laboratory Report entitled "ELMO Bumpy Square Status Report" dated Jan. 1984 and compiled by N. A. Uckan and in a publication of the proceedings of a workshop entitled "Advanced Bumpy Torus Concepts" Conf.-830758, edited by N. A. Uckan and published October 1983. The subject matter of these references being incorporated herein by reference thereto.
Optimizing the design of an EBT requires a magnetic field configuration that maximizes particle confinement within the toroidal volume. The toroidal curvature of the magnetic field in an EBT results in an inward shift of the particle drift orbits toward the major axis thereby reducing the average length of time particles are confined by the magnetic field. Particles with different components of velocity parallel to the magnetic field are shifted inward by different amounts. The resulting dispersion in drift orbits enables particles to diffuse or random walk out of the plasma by coulomb collisions with other particles. This plasma loss is minimized by minimizing the dispersion in drift orbits that results from the toroidal curvature of the magnetic field. In the standard EBT (toroidal) geometry the only way to reduce the plasma loss and maximize the particle confinement time is to make the device larger.
Thus, there is a need for an improved geometric configuration of an Elmo bumpy type plasma confinement device which takes advantage of the desirable characteristics of this type of confinement while eliminating the detrimental effects of a conventional toroidally closed plasma confinement device of the Elmo bumpy type, i.e., the EBT.